I'm using the Mail application bundled with OS X 10.8. When switching from Mac OS X 10.6, I noticed that Mail now always includes a text/html part, alongside the text/plain part, in messages that include an image as attachment, even if the message's text has not been formatted in any way. Adding a different file type as attachment does not show that behavior.

The second problem is that the attached image is contained in the text/html part and not the text/plain part. Recipients whose mail clients (by configuration or by default) display the text/plain part do not see the attached image, which is very annoying.

Is there a way to make Mail never include a text/html part, by configuration or by using a specific method to insert the attachment?

text/plain cannot show an image so what do you expect the email to consist of?
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MarkFeb 14 '13 at 10:55

@Mark Of course it can! You can use a content-type of multipart/mixed and set the content-disposition of the part with the image to inline (see RFC 1341 and RFC 2183). This will result in what is often called an "inline attachment". This works independent of the content-type parts containing the message's text. This is what was happening in earlier versions of the Mail application.
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FeuermurmelFeb 14 '13 at 14:35